316 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
79. The protein standard for certified cream shall corre- 
spond to the protein standard for certified milk. 
80. The protein content shall be determined only when any 
special consideration seems to the medical milk commission to 
make it desirable. 
81. It shall be determined by the Kjeldahl method, using 
the Gunning or some other reliable modification, and employing 
the factor 6.25 in reckoning the protein from the nitrogen. 
Kjeldahl Method.—Five cubic centimetres are measured 
carefully into a flat-bottom 800 c.c. Jena flask, 20 c.c. of concen- 
trated sulphuric acid (C. P.; sp. gr., 1.84) are added, and 0.7 
gramme of mercuric oxide (or its equivalent in metallic mer- 
cury); the mixture is then heated over direct flame until it is 
straw-colored or perfectly white; a few crystals of potassium 
permanganate are now added till the color of the liquid remains 
green. All the nitrogen in the milk has then been converted 
into the form of ammonium sulphate. After cooling, 200 c.c. 
of ammonia-free distilled water are added, 20 c.c. of a solution of 
potassium sulphide (containing 40 grammes sulphide per litre), 
and a fraction of a gramme of powdered zinc. A quantity of 
semi-normal HC] solution more than sufficient to neutralize the 
ammonia obtained in the oxidation of the milk is now carefully 
measured out from a delicate burette (divided into 1/20 c.c.) 
into an Erlenmeyer flask and the flask connected with a distil- 
lation apparatus. At the other end the Jena flask containing 
the watery solution of the ammonium sulphate is connected, 
after adding 50 c.c. of a concentrated soda solution (1 pound 
‘pure potash” dissolved in 500 c.c. of distilled water and allowed 
to settle); the contents of the Jena flask are now heated to 
boiling, and the distillation is continued for 40 minutes to an 
hour, until all ammonia has been distilled over. 
The excess of acid in the Erlenmeyer receiving flask is then 
accurately titrated back by means of a tenth-normal standard 
ammonia solution, using a cochineal solution as an indicator. 
From the amount of acid used the per cent. of nitrogen is ob- 
tained; and from it the per cent. of casein and albumen in the 
milk by multiplying by 6.25. The amount of nitrogen contained 
in the chemicals used is determined by blank experiments and 
